1. Field of the Invention
The present invention relates to hard-disk drive (HDD) systems, and, in particular, to controlling fly height in such systems.
2. Description of the Related Art
In hard-disk drive (HDD) systems, the spacing between (i) the element of a read/write head and (ii) the surface of the platter, commonly referred to as fly height, is an important parameter affecting both capacity and performance. In general, reducing fly height during writing and/or reading operations improves bit-error rate. However, reducing the fly height too significantly can result in the read and/or write element contacting the surface of the platter. Such contact is typically undesired because it may corrupt data stored on the platter and possibly even damage the platter and/or heads.
Typically, fly height is set during manufacturing. However, during operation, fly height may change due to environmental conditions such as temperature and/or voltage variations. For example, when a write operation is performed, a write current is applied to the head. Note that the read and write elements are typically co-located on the same head, and that, reading and writing may be performed using a single read/write element or separate read and write elements. For the following discussion, the term “head” refers to a component that may have any of these configurations. The write current may cause the write element to heat up, and as a result, the material of the write element may expand such that the write element protrudes from its initial orientation towards the surface of the platter. When the write operation ceases, the write element cools and the material of the write element contracts such that the write element returns to (or near) its initial orientation. HDD system designers have expended a substantial amount of effort attempting to compensate for read and/or write element protrusion. Discussion of some of these efforts may be found in, for example, Schultz, “Thermal Fly-height Control (TFC) Technology in Hitachi Hard Disk Drives,” www.hitachi.com, and Tang, “Overview of Fly Height Control Applications in Perpendicular Magnetic Recording,” IEEE Transactions on Magnetics, Vol. 43, No. 2, February 2007, the teachings all of which are incorporated herein by reference in their entirety.
FIG. 1 shows a simplified flow diagram 100 of a prior-art method for adjusting fly height. During manufacturing, two periodic patterns, which are used during operation of the HDD system for detecting change in fly height, are written to specified locations on the platter (action 102). The two periodic patterns may be written to the platter as one recorded pattern in which the two periodic patterns are combined, or as two separate recorded patterns. For example, one recorded pattern, which is constructed from (i) a first periodic pattern having a first frequency f1 (e.g., a first harmonic of a periodic pattern) and (ii) a second periodic pattern having a second frequency f2 (e.g., a third harmonic of a periodic pattern), may be written to a single track of the platter. Alternatively, a first pattern having a first frequency f1 may be written to a first track of the platter, and a second pattern having a second frequency f2 may be written to a second track of the platter.
Further, during manufacturing (e.g., calibration), an operating fly height d0 is selected (action 104) using any suitable fly-height selection technique. Such a technique might involve touchdown of the head to the surface of the platter. Along with selecting an operating fly height d0, a corresponding harmonic ratio R0 (action 104) is determined at the operating fly height d0. Harmonic ratio R0 may be represented as shown in Equation (1):
                                          R            0                    ⁡                      (                                          k                1                            ,                              k                2                                      )                          =                              ln            ⁡                          (                                                                    V                    0                                    ⁡                                      (                                          k                      1                                        )                                                                                        V                    0                                    ⁡                                      (                                          k                      2                                        )                                                              )                                =                                                    (                                                      k                    2                                    -                                      k                    1                                                  )                            ⁢                              d                0                                      +                                          B                0                            ⁡                              (                                                      k                    2                                    ,                                      k                    1                                                  )                                                                        (        1        )            where k1 and k2 are the wave numbers (i.e., k=2π/λ) corresponding to frequencies f1 and f2, respectively, λ denotes wavelength, V0(k1) and V0(k2) are the read-back signal strengths corresponding to frequencies f1 and f2, respectively, and
                    B        0            ⁡              (                              k            1                    ,                      k            2                          )              =          ln      ⁡              (                                            A              0                        ⁡                          (                              k                1                            )                                                          A              0                        ⁡                          (                              k                2                            )                                      )              ,where A0(k1) and A0(k2) are the channel gains corresponding to frequencies f1 and f2, respectively. Further, Equation (1) assumes that read-back signal strengths V0(k1) and V0(k2) may be represented by the Wallace spacing loss equation as follows:Vt(ki)=At(ki)e−kidt  (2)where subscript i indicates the periodic pattern read back (i.e., i=1 and i=2 for the patterns corresponding to frequencies f1 and f2, respectively), and subscript t indicates the point in time that signal Vt(ki) is read back (i.e., t=0 during calibration). Note that, according to the Wallace equation, for a given wave number ki (i.e., frequency), the read-back signal strength Vt(ki) increases as the fly height dt decreases.
After manufacturing, and during operation of the HDD system (i.e., at a time other than t=0), fly-height measurements may be initiated using any suitable trigger. For example, fly-height measurements may be performed based on a change of temperature, a change in performance of the HDD system, or after a specified amount of time has elapsed. Once a fly-height measurement is initiated (decision 106) (e.g., at time t>0), the HDD system measures the read-back signal strength Vt(k1) at the first frequency f1 and the read-back signal strength Vt(k2) at the second frequency f2, both of which may be represented as shown in Equation (2) (action 108).
A harmonic ratio Rt is calculated (action 110) based on the first and second read-back signal strengths Vt(k1) and Vt(k2), and may be represented as shown in Equation (3) below:
                                          R            t                    ⁡                      (                                          k                1                            ,                              k                2                                      )                          =                              ln            ⁡                          (                                                                    V                    t                                    ⁡                                      (                                          k                      1                                        )                                                                                        V                    t                                    ⁡                                      (                                          k                      2                                        )                                                              )                                =                                                    (                                                      k                    2                                    -                                      k                    1                                                  )                            ⁢                              d                t                                      +                                          B                t                            ⁡                              (                                                      k                    2                                    ,                                      k                    1                                                  )                                                                        (        3        )            where time t>0, Vt(k1) and Vt(k2) are the read-back signal strengths corresponding to frequencies f1 and f2, respectively, and
                    B        t            ⁡              (                              k            1                    ,                      k            2                          )              =          ln      ⁡              (                                            A              t                        ⁡                          (                              k                1                            )                                                          A              t                        ⁡                          (                              k                2                            )                                      )              ,where At(k1) and At(k2) are the channel gains corresponding to frequencies f1 and f2, respectively. Note that, in Equation (3), Rt(k1, k2) may be determined from Vt(k1) and Vt(k2), and that both fly height dt and Bt(k1, k2) are unknown.
To eliminate unknown Bt(k1, k2) from Equation (3), it may be assumed that Bt(k1, k2)=B0(k1,k2). Then, a difference between harmonic ratios Rt(k1, k2) and R0(k1,k2) may be calculated (action 112) by subtracting Equation (1) from Equation (3) as follows in Equation (4):ΔR=Rt(k1,k2)−R0(k1,k2)=(k2−k1)Δd  (4)where, Δd=dt−d0. Further, Equation (4) may be rewritten as shown in Equation (5):
                              Δ          ⁢                                          ⁢          R                =                                                            2                ⁢                π                            v                        ⁢                          (                                                f                  2                                -                                  f                  1                                            )                        ⁢            Δ            ⁢                                                  ⁢            d            ⁢                                                  ⁢            and            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢                          R              ⁡                              (                dB                )                                              =                                                    2                ⁢                π                            v                        ⁢                          (                                                f                  2                                -                                  f                  1                                            )                        ⁢            Δ            ⁢                                                  ⁢                          d              ·                              20                                  ln                  ⁡                                      (                    10                    )                                                                                                          (        5        )            where ν=the linear velocity of the platter under the head.
From Equations (4) or (5), the change in fly height αd may be determined, and this change may be used to adjust the fly height (action 114) as appropriate. For example, in one common method, an electrically resistive heating element may be located on the head to control expansion and contraction of the write and/or read elements. This heating element may be controlled by a thermal actuator that increases or decreases the heat applied by the heating element based on the detected change in fly height Δd to expand or contract the material of the write and/or read elements. In other common methods, other fly-height actuators such as (i) electrostatic microactuators, (ii) piezoelectric actuators, and (iii) fly-height actuators, based on thermal, electrostatic, or piezoelectric techniques, that alter the air-flow or the shape of the air-bearing slider may be used to control fly height. Note that, since f1 and f2 (and similarly k1 and k2) are constant, the controller may adjust fly height based on ΔR without separately computing Δd. This process may be repeated (decision 116), where each repetition is triggered based on a suitable criterion such as one of those discussed above in relation to decision 106.
One problem with the method of FIG. 1 is that Bt(k1, k2) might not be equal to B0(k1, k2). Bt(k1, k2) and B0(k1, k2) may change based on, for example, environmental variations that affect the electronics in the HDD system read path. Deviations between Bt(k1, k2) and B0(k1, k2) may lead to inaccuracies in ΔR in Equations (4) and (5) and, consequently, to inaccuracies in the measured change in fly height Δd. To meet relatively demanding requirements for fly-height accuracy, ΔR should preferably vary only within, for example, a few tenths of a dB over the complete range of variation in environmental conditions (e.g., voltage and temperature). To overcome these inaccuracies, a method for measuring change in fly height Δd is needed that is not affected by these variations in environmental conditions.